Cold spray is continuously expanding for the repair of parts made of aluminum-based alloys. Beyond repair applications, the process is now expected to be exploited efficiently for the additive manufacturing of shaped parts. However, up to now, cold spray is limited to the achievement of rather simple shapes due to a lack of basic knowledge on coating build-up mechanisms to result in dimension-controlled deposition. The objective of this work is to fill that gap through an experimental and modeling study of the coating build-up in cold spray for this specific application. Experimentally, Al-based coatings were deposited for a large range of particle velocity due to the use of low-pressure, medium-pressure and high-pressure cold spray facilities. Particle velocity was monitored as a function of cold spray conditions. Two different types of Al 2024 (Aluminium 2024 Alloy) powders were tested. Coating porosity and microhardness were studied as a function of (both morphological and metallurgical) powder characteristics and spray conditions, primarily in the light of particle velocity. Various correlations could be exhibited. Finite element (FE) simulations of particle impacts were developed, including particle velocity from experimental measurements. These will be used as inputs in an in-house morphological model, the first stages of which could be established successfully.

In this study, a hybrid plasma spraying process is used to produce particle-reinforced metal-matrix composite coatings on 316 stainless steel. Two injectors are mounted at the output of the plasma gun, one feeding a nickel-base alloy powder, the other feeding a suspension of alumina nanoparticles. Different feed rates, suspension compositions, and alumina particle contents are used and their effects on microstructure, microhardness, porosity, adhesion, and wear behavior are assessed.

In nuclear plants, the replacement of hardfacing Stellite, a cobalt-base alloy, on parts of the piping system in connection with the reactor has been investigated since the late 60’s. Various Fe-base or Ni-base alloys, Co-free or with a low content of Co, have been developed but their mechanical properties are generally lower than that of Stellites. The 4th generation nuclear plants impose additional or more stringent requirements for hardfacing materials. Plasma transferred arc (PTA) coatings of cobalt-free nickel-base alloys with the addition of sub-micrometric or micrometric alumina particles are thought to be a potential solution for tribological applications in the primary system of sodium-cooled fast reactors. In this study, PTA coatings of nickel-base alloys reinforced with alumina particles were deposited on 316L stainless steel substrates. The examination of coatings revealed a refinement of the microstructure. Under the conditions of the study, the addition of alumina particles did not improve the micro-hardness of coatings but improve their resistance to abrasive wear.

In this work, a numerical model of the cold spray process was developed to reproduce microstructures obtained in coatings by simulating the deformation of impinging particles and resulting coating build-up. The model employs a library of particle images generated by x-ray microtomography. To each image, a velocity is assigned and the deformation that would be produced by particle impact is estimated by means of finite element analysis and stored for later use. Based on the results, the ing approach has good potential for simulating coating microstructures that can be achieved through cold spraying.

In the present study, X-ray microtomography is used to examine cold-sprayed tantalum splats on copper substrates. To resolve tantalum splats intermeshed with other splats of the same chemical composition, a contrasting medium of some sort is required. For this purpose, the feedstock powder is coated with an iron layer by means of fluidized-bed chemical vapor deposition. Experimental tests were coupled with finite element simulations to determine how stresses generated during the impact of a spherical iron-coated particle affect the integrity of the added contrasting layer.

Materials with fine-scale structures are recognized to exhibit technologically efficient mechanical or thermal properties. Thermal sprayed nanostructured media could also be interesting in the optical field thanks to the diffusion interaction between short wavelength radiations and submicron-sized pores. One difficulty of the Suspension Plasma Spray (SPS) technique is to provide a sufficiently thick coating and to maintain efficient mechanical properties when the coating is removed from the substrate. The aim of this study was to spray a reflective and diffusive self-supporting alumina bilayer. The first coating was obtained by Atmospheric Plasma Spraying of micron-sized powders in order to obtain a thick layer and the second coating was sprayed by SPS onto the first one with strong adhesion. An oxide ceramic material with high scattering reflectance in a large band of wavelengths was thus obtained.

The adhesion of plasma-sprayed coating is to a large extent controlled by the cleanness and roughness of the surface on which the coating is deposited. So, most of the plasma spray procedures involve surface pretreatment by grit-blasting to adapt the roughness of the surface to the size of the impacting particles. This preparation process brings about compressive stresses that make it inappropriate for thin substrates. The present works aims to elaborate a ceramic coating on a thin metal substrate with a smooth surface. The coating system is intended for use in a generation–IV nuclear energy system. It must exhibit a good adhesion between the ceramic topcoat (about 0.5-mm thick) and the smooth metal substrate (1-mm thick) to meet the specifications of the application. Our approach has consisted in depositing the ceramic layer on a few micrometers thick ceramic layer made by suspension spraying. We have observed the interface between both ceramic layers by transmission electronic microscope and studied the adhesion of the nanostructured layer by the Vickers Indentation Cracking technique and that of the coating system by tensile test.

A new challenge in the field of Solid Oxide Fuel Cells (SOFCs) concerns the reduction of their operating temperature down to 700°C. Apatite ceramics are interesting alternatives for SOFC electrolytes due to their high ionic conductivity at this temperature. The present work reports on the manufacturing and characterization of La 9 SrSi 6 O 26.5 coatings obtained by atmospheric plasma spraying at two different plasma spray powders. The microstructure and the composition of the as-sprayed and heat-treated coatings were investigated by several techniques including X-Ray Diffraction, Inductively Coupled Plasma - Atomic Emission Spectroscopy as well as Scanning and Transmission Electron Microscopy. The porosity level of the coatings was evaluated by the Archimedean method and image analysis. The studies revealed that the as-sprayed apatite coatings were composed of an amorphous phase, a crystalline apatite phase and chemical heterogeneities due to Si volatilization in the high-temperature plasma. Furthermore, a heat treatment made it possible to obtain denser, fully crystallized apatite coatings and also improved their ionic conductivity.

The gas-cooled fast reactor is a 4th generation nuclear reactor currently under development. Its design concept requires protective coatings able to operate at 850°C and protect the underlying structure in case of extreme cases, where the functional temperature can increase up to 1250°C and there is depressurization from 70 bars to atmospheric pressure. The parts to be covered are made in 1-mm thick materials resistant to heat and erosion with high mechanical properties at high temperatures, such as the Haynes 230 nickel-based alloy. In this study, the potential of the suspension plasma spraying technique for forming the first layers of a ceramic coating on smooth 1-mm thick Haynes substrate was explored. In order to meet these specifications, the coating material selected was partially stabilized zirconia of standard composition (8 mol.% Y 2 O 3 -ZrO 2 ).

This study investigates the feasibility of manufacturing lanthanum silicate coatings for solid oxide fuel cells by atmospheric plasma spraying. The coatings produced are cohesive, relatively dense, and contain no secondary phases. Test results show that the starting composition of the powder is maintained after spraying and that particle velocity depends primarily on the plasma gas mixture. Due to the relatively high melting temperature of the particles, dense apatite coatings with few microstructural defects are obtained.

The peculiar microstructure of plasma-sprayed coatings affects their physical properties. Many of these properties are, to a great extent, dominated by the influence of the spraying process on in-flight particle parameters. In order to improve the quality of plasma-sprayed coatings and to enhance the efficiency of plasma spray deposition, a wide range of anode nozzles have been developed for air plasma torches. The objective of this work is to investigate the influence of two convergent-divergent nozzle designs on plasma-sprayed YSZ particles at impact and the permeability, porosity, and microstructure of the resulting coatings.

Suspension plasma spraying (SPS) is of great interest for the realization of thin nanostructured coatings. The thermokinetic phenomena involved in the SPS process, from the plasma/liquid interaction to the growth of the coating are difficult to forecast and experiment. The present work focuses on the 3D simulation of an Ar-H 2 (45-15 slm) plasma jet issuing into ambient air. A large eddy simulation model accounts for turbulence and provides us with the instantaneous temperature and velocity fields of the plasma flow. The results obtained with the CFD library AQUILON are compared to experimental ones as well as previous simulations realized with a commercial software.

The objective of this study is to establish relationships between the optical properties of yttria-stabilized zirconia (YSZ) plasma-sprayed coatings and their microstructure to improve YSZ thermal barriers. So, coatings with significant microstructural differences were manufactured and their hemispherical reflectance and transmittance were investigated over the 0.25-20µm wavelength range. However as plasma-sprayed coatings are heterogeneous materials, they exhibit optical properties different from those of homogeneous ones. Indeed, the pores and grain boundaries act as scattering centers and cause modifications in optical properties that are closely linked both to intrinsic properties of materials and microstructural characteristics. Therefore, the extinction coefficient which describes the decrease in light intensity due to the light absorption and diffusion processes in the matter, was determined by using the Kubelka-Munk two-flux model. The absorption and diffusion coefficients were infered from the comparison between the experimental data and model predictions of reflectance and transmittance.

D.C. plasma spraying process leads to coatings with high level properties. The control of these properties implies to overcome well the process and in particular the interactions between the particles and the plasma plume. This latter shows high fluctuations outside the torch due to arc root movements on the anode wall inside the torch. These movements are responsible of less or more weak plasma flow stretching outside the torch and of air engulfment. From a simple numerical model based on the gas heating by JOULE effect in the anode, the realistic flow behavior is reproduced, in particular, the mechanisms of air engulfment into the plasma plume, such as it was described by PFENDER and al. Moreover, the centerline velocities and temperatures of the gas are compared with experimental values: the comparison shows good agreement. From this realistic flow, zirconia particle treatment modeling is carried on. Depending on the plasma plume stretching outside the torch and on particle granulometry, the particle trajectories and also velocities and temperatures present high variations, leading to particle thermocinetic states at impact quite different from one particle to the other.

This work deals with a 3-D time-dependent modeling of the arc behavior in a plasma spray torch. The mathematical model is based on the simultaneous solutions of the conservation equations of mass, momentum, energy, electric current and the electromagnetism equations. It makes it possible to predict the motion of the anode attachment root on the anode surface under the combined effect of hydrodynamic and magnetic force and, the time-evolution of arc voltage and gas fields in the nozzle. The calculations show that the latter exhibit significant three-dimensional characteristics. The projected arc behavior and voltage fluctuations agree rather well with experimental observations.

Inductive plasma torches are used in a wide range of applications such as spraying, waste destruction, powder treatment... In this kind of torch, the gas is heated to plasma state by ohmic heating due to the inductive current created by an electromagnetic field. Modeling this phenomenon has been studied by S. Xue and implemented in the commercial code FLUENT. This model is improved by introducing a turbulence model. To validate the model, experimental investigations are carried on. The gas temperature is measured using an enthalpy probe inside a tubular reactor at different positions. The measurements are compared with computational results and the accuracy is rather good.

This paper presents the numerical simulation of the plasma flow into a dense atmosphere. The plasma generation is performed by the simple model developed by Eichert. Two models are exposed to take in account the arc fluctuation inside the anode. They permit us to simulate plasma puffs convected into the flow. The aim of this study is to compare these two models with experiments and to determine which one is the most relevant.

Relationships between the electrical properties of thermally sprayed titania coatings and their microstructure have been investigated. As far as possible, a broad range of microstructures was produced by using various processes of plasma spraying with different powder size ranges and variations of the plasma operating parameters. The two spraying processes consisted of DC plasma spraying and RF plasma spraying. Physical properties of plasma-sprayed coatings are generally influenced by their microstructure. But the electrical properties of plasma-sprayed titania coatings are known to be strongly influenced by their stoichiometry. It is the reason why coatings with identical stoichiometry were compared. It was found that electrical resistivity was directly linked to the quality of the contact between the splats and their density through the titania plasma-sprayed coatings.

Production of ceramic coatings by plasma spraying on thermally sensitive substrates requires a hard cooling onto the substrate immediately after the plasma and particles jet getting-through. That implies a cryogenic cooling spray. In order to have a better understanding of thermal transfer between cryogenic droplets and substrate, we simulate with a home-made software, PARTIX, the transport of pulverized jet in the atmosphere and calculate the droplet size distributions impacting the substrate. Knowing that, we can determine the thermal flux. Simulations of different pulverized flows are presented.

In dc plasma spray guns, the properties of the plasma forming gas largely control the characteristics of the plasma jet and the momentum and heat of the particles injected into the flow. This paper examines the effect of Ar-He-H2 mixtures on the dynamic and static behavior of plasma jets expressed in terms of arc voltage and gas velocity. Correlations between these parameters and operating variables (arc current, gas flow rate, volume composition) were established from a dimensional analysis and supported by the calculation of the thermodynamic and transport properties of the gas mixtures used in the study.